There is a need for multilayer heat-shrinkable films and articles of manufacture made therefrom, which have high impact strength, especially at elevated temperatures, high free shrink at 185° F., high modulus, high gloss and package presentation, good sealability and seal strength, and stack/overlap sealing capability, and which can be easily oriented. This combination of features is not currently available.
Recently it has been discovered that certain commercially available bags can be sealed when stacked on top of one another, i.e., without sticking to one another. This non-sticking characteristic provides an advantage for packaging in a vacuum chamber, because the chamber, although typically having only one sealing means, has more than enough space therewithin for multiple bagged products which are to be sealed after evacuation of the atmosphere from the chamber. Thus, the non-sticking feature enables the evacuation and sealing of more than one bag at a time in a vacuum chamber, thereby increasing the production rate of the vacuum chamber packaging apparatus.
U.S. Pat. No. 5,336,549, to Nishimoto et al., discloses a heat-shrinkable film which can be made into bags. Apparently, users of this film, which is commercially available, have discovered that bags made from the film can be stacked on top of one another during sealing, without sticking to one another (i.e., the bags are “stack-sealable”). This enables the output of vacuum chamber packaging machinery to be, for example, doubled, if two bags are stacked on top of one another and simultaneously sealed.
The film disclosed in the '549 patent has an outer layer of a polyester, and an intermediate layer of a polyamide having a melting point of higher than 160° C. and lower than 210° C. In addition, the '549 patent goes on to show that if the melting point of the polyamide is too low (e.g., 135° C. in Comparative Example 2), the variation in the load of the extruder was great and the stretchability of the film is unstable, resulting in a film having too much dimensional variation, and a thickness which was too lacking in uniformity. In Comparative Example 4, the '549 patent states that stretching was impossible if a polyamide having a melting point as high as 265° C. was used.
Although Nishimoto et al discloses a large group of polyamides for use in an inner layer, together with various polyesters for use in an outer layer, Nishimoto et al teaches that a polyamide having a melting point below 160° C., if used alone in the inner layer, results in a great variation in the load on the extruder, variation in the inflation initiating point, and shrinkage of at least 8% during storage at room temperature. Nishimoto et al does disclose a film using a blend of a high melting polyamide and a low melting polyamide, but the blend contained only 30% of the low melting polyamide.